cos (α + β) = cos α cos β — sin α sin β
y=cos(x+pi/3)=cos x cos π/3 - sin x sin π/3 = ½cos а - (√3/2)*sin а
при а=π/3 у = ½cos π/3 - (√3/2)*sin π/3 = 1/4 - 3/4 = -0,5
при а=π/6 у = ½cos π/6 - (√3/2)*sin π/6 = √3/4 - √3/4 = 0
